Motor vehicle suspension systems are configured so that the wheels are able to follow elevational changes in the road surface as the vehicle travels therealong. When a rise in the road surface is encountered, the suspension responds in “jounce” in which the wheel is able to move upwardly relative to the frame of the vehicle. On the other hand, when a dip in the road surface is encountered, the suspension responds in “rebound” in which the wheel is able to move downwardly relative to the integrated body/frame structure of the vehicle. In either jounce or rebound, a spring (i.e., coil, leaf, torsion, etc.) is incorporated with the body structure in order to provide a resilient response to the respective vertical movements of the wheel with regard to the vehicle body structure. However, in order to prevent wheel bouncing and excessive vehicle body motion, a shock absorber or strut is placed at the wheel to dampen wheel and body motion. An example of a MacPherson strut is disclosed in U.S. Pat. No. 5,467,971.
Of interest is a prior art single fastener strut top mount for a MacPherson strut which is manufactured by Adam Opel GmbH, a division of General Motors Corporation, Detroit, Mich., and is shown generally at 10 in FIG. 1. This prior art strut top mount 10 interfaces with a broad, annular strut tower 12 which at its lower end (not shown) is connected to the body structure of the motor vehicle. This prior art strut top mount 10 features an annular tapered dome 14 that is open downward nestingly within the tower 12, and is welded thereto at a conjoining 16 (the taper being smallest adjacent the conjoining, and largest distant from the conjoining). An annular outer rubber element 18 has an inclined outer surface 18a which abuts the dome 14. An annular metal insert 20 is preferably provided, for stiffening, within the outer rubber element 18 adjacent the dome 14. An annular stamped metal support shell 22 is adhered to the outer rubber element 18 in nested (i.e., in cross-section being oppositely disposed) relation to the dome 14; and an annular inner rubber element 24 is nested within and adhered to the support shell 22 in cross-section being in opposite disposition with respect to the outer rubber element 18, wherein the aforementioned adherences result from the molding process of the inner and outer rubber elements.
At an annular shelf 22a of the support shell 22, within an upper polymer housing 26c, is an upper race 26a of an annular bearing 26. The lower race 26b of the bearing 26, within a lower polymer housing 26d, is located at an annular spring bracket 28, wherein the upper and lower polymer housings mutually have a conventional labyrinthine seal interfacing, and wherein the spring bracket locates and handles loads from both the coil spring 32 and the jounce bumper 34. At an outer periphery 28a of the spring bracket 28, wherein the spring bracket has a diameter less than that of the strut tower 12, but exceeding the diameter of the dome 14, is formed a spring seat 30 having a rubber insulator 30a upon which abuts the coil spring 32. At an inner periphery 28b of the spring bracket 28, adjacent the bearing 26, is a connection 28c to the jounce bumper 34. A strut shaft 36 is reciprocally interfaced to a strut housing (not shown) in a conventional manner so as to provide damping as it reciprocates in relation thereto in response to jounce and rebound. A tubular metal sleeve 35 receives the strut shaft 36 at a shoulder 36a thereof, wherein the sleeve is adhered (as a result of the aformentioned molding process) to the inner rubber element 24. At the shoulder 36a of the strut shaft 36 is a lower washer 38 which abuts a lower end 24a of the inner rubber element 24 and a lower end of the sleeve 35. Abutting an upper end of the sleeve 35 is an upper washer 40 which also abuts an upper end 24b of the inner rubber element 24, wherein the upper washer is held in place by a first nut 42 that is threaded onto the strut shaft 36. A retention washer 44 is mounted onto the strut shaft 36, and is held in place between the first nut and a second nut 46, which is also threaded onto the strut shaft. At the periphery of the retention washer 44 is a retention washer rubber element 48.
FIGS. 1A and 1B are graphs showing what is believed to be the response of the prior art strut top mount 10 to jounce and rebound. In this regard, FIG. 1A shows a graph 50 of load force versus displacement in which plot 52 indicates the believed response of the outer rubber element 18 to jounce; and FIG. 1B shows a graph 60 of load force versus displacement in which plot 62 indicates the believed response of the prior art strut top mount 10 to jounce and rebound.
From FIGS. 1A and 1B, several conclusions can be drawn with respect to the prior art strut top mount 10. Asymmetry is seen in the jounce to rebound rate ratio. Over-travel of the prior art strut top mount 10 can occur if the jounce bumper forces are routed through the inner rubber element, wherein the prior art strut top mount would have an unacceptably large amount of travel in jounce; and is likely why the jounce bumper forces are instead routed through the spring loadpath. The latter loadpath arrangement does not enable use of monotube struts (monotube struts route the jounce bumper loads up through the strut rod and, therefore, the strut top mount thereof requires an appropriately high load capacity in the damper rod load-path to handle them, whereby this is possible only for a single path strut top mount or a dual path strut top mount with the bumper and damper loads combined into the same path). The rebound rate changes abruptly as the retention washer rubber element engages the strut tower. The outer rubber element axial rate cannot be used to tune the prior art strut top mount 10 axial rate range, and the inner rubber element has little authority over overall axial rate, wherein the outer rubber element axial rate is determined by the inherent deflection requirement for retention (the retention objective is to ensure that the prior art strut top mount maintains contact with the underside of the strut tower at all times, and the outer rubber element compliancy allows for the retention washer to separate from the strut tower and thereby create freedom of mount movement during operation). With respect to the radial rate range of the prior art strut top mount 10, the large outer rubber element acts in series with the inner rubber element and is relatively soft in the radial direction. Additionally, the design height position of the prior art strut top mount 10, when loaded under vehicle curb weight, varies with vehicle mass, making it difficult to use this design on multiple vehicle applications (i.e., differing rubber chemistry or durometer of each of the inner and outer rubber elements is required, wherein the outer rubber element is adjusted to vehicle mass to provide a desired retention washer gap with respect to the strut tower, and the inner rubber element is tuned to adjust, somewhat, the mount rate).
Accordingly, what remains needed in the art is a single fastener, compact strut top mount which has an optimized resilient response to jounce and rebound.